1. Field of the Invention
The present invention relates generally to a position detecting apparatus for detecting a position of a substrate or the like as an object to be detected and, more particularly, to an apparatus suitable for detecting a focal point of, e.g., a microscope.
2. Related Background Art
A known focus detecting apparatus employed for a fall illumination type microscope is disclosed in, e.g., U.S. Pat. No. 3,721,827.
Now, the focus detecting apparatus disclosed in U.S. Pat. No. 3,721,827 will be described with reference to FIGS. 14A through 14C. Herein, FIG. 14A depicts an in-focus state where a detected surface 70 coincides with an object plane P.sub.0 (fiducial plane) of an objective lens 6. FIG. 14B illustrates a defocus state where the detected surface 70 deviates downward from the object plane P.sub.0 (fiducial plane) of the objective lens 6. FIG. 14C illustrates a defocus state where the detected surface 70 deviates upward from the object plane P.sub.0 (fiducial plane) of the objective lens 8.
To start with, as depicted in FIG. 14A, the light emitted from a light source 1 is condensed by a condenser lens 2. A slit plate 3 formed with a slit opening is illuminated with the light, thus forming a slit illuminant. Then, the beam of light is split into two subbeams by a plane (perpendicular to the sheet surface of the figure) including an optical axis Ax.sub.1 of the condenser lens 2. A light intercepting plate 4 intercepts one subbeam, a half (lower subbeam), of the beam through the slit plate 3. The other subbeam L.sub.1, a half (upper subbeam), of the beam is reflected by a half-mirror 5. Thereafter, the beam L.sub.1 reflected by the half-mirror 5 penetrates a left half (left half of the pupil of the objective lens 6) of an objective lens 6 and is converged on a detected surface 70 (a fiducial object plane P.sub.0 of the objective lens 6).
An image of the opening of the slit plate 3 is formed on this detected surface 70. A beam of light L.sub.2 reflected by the detected surface 70 passes through a right half (right half of the pupil of the objective lens 5) of the objective lens 6. Thereafter, the beam L.sub.2 penetrates the half-mirror 5 and is converged on the light receiving surface of the photoelectric detector 8. Herein; an image of the opening of the slit plate 3 is formed.
The light receiving surface of the photoelectric detector 8 has light receiving areas 8a, 8b formed right and left of the boundary plane which includes an optical axis Ax.sub.2 of this objective lens 6. An imaged state of the opening of the slit plate 3 is thus photoelectrically detected.
Further, as shown in FIG. 14B, the detected surface 70 is located in a position P.sub.1 lower than a position P.sub.0 of an object plane (fiducial plane) of the objective lens 6. In this case, the beam L.sub.2 reflected by the detected surface 70 and penetrating the right half of the objective lens 6 is converged in front of the light receiving surface of the photoelectric detector 8. The beam L.sub.2 then reaches the left area 8a of this light receiving surface. Herein, a slit image is formed in a converging position A.sub.0 in front of the light receiving surface of the photoelectric detector 8. Therefore, this photoelectric detector 8 photoelectrically detects a defocus image of the slit 3.
Further, as depicted in FIG. 14C, the detected surface 70 is located in a position P.sub.2 higher than the position P.sub.0 of the object plane (fiducial plane) of the objective lens 6. In this instance, the beam reflected by the detected surface 70 and penetrating the right half of the objective lens 6 reaches the right area 8b of the light receiving surface so that the beam is converged in rear of the light receiving surface of the photoelectric detector 8. Then, the photoelectric detector 8 photoelectrically detects a defocus image of the slit 3.
Herein, the focus detection through the photoelectric detector 8 is conducted by taking a balance, i.e., a difference, between two quantity-of-light signals detected respectively by the right-and-left light receiving areas 8a, 8b of the photoelectric detector 8.
For instance, as illustrated in FIG. 14A, when in focus, the detection light is detected at the center of the light receiving surface of the photoelectric detector 8. For this reason, outputs of the two quantity-of-light signals are equal, these signals being detected respectively by the right-and-left light receiving areas 8a, 8b of the light receiving surface. A differential signal is thereby zeroed.
As shown in FIG. 14B, when the detected surface 70 is in a defocus state with a downward deviation from the object plane P.sub.0 (fiducial plane) of the objective lens 6, the detection light is detected mainly by the left area 8a of the light receiving surface of the photoelectric detector 8. For this reason, an output signal assuming, e.g., a certain positive level is obtained with respect to the differential signal between the two quantity-of-light signals detected individually by the right-and-left light receiving areas 8a, 8b of the light receiving surface.
Besides, as illustrated in FIG. 14C, when the detected surface 70 is in a defocus state with an upward deviation from the object plane P.sub.0 (fiducial plane) of the objective lens 6, the detection light is detected chiefly by the right area 8b of the light receiving surface of the photoelectric detector 8. Accordingly, an output signal assuming, e.g., a certain negative level is obtained with respect to the differential signal between the two quantity-of-light signals detected respectively by the right-and-left areas 8a of the light receiving surface.
In this manner, an out-of-focus direction can be detected depending on the negative or positive level of the differential signal by taking a difference between the two signals obtained from the photoelectric detector 8. An out-of-focus quantity can be detected in accordance with the output level of the differential signal.
By the way, in recent years, there has rapidly increased a rate of the liquid crystal display spreading in the sector of display devices, etc. It is therefore of importance to inspect a liquid crystal substrate as a base of the liquid crystal display. This liquid crystal substrate is examined mainly by a microscope. In an attempt at focusing on the surface of the liquid crystal substrate by use of the microscope incorporating the above-mentioned focus detecting apparatus, however, the photoelectric detector receives not only the reflected light from the liquid crystal substrate which is to be originally detected but also the reflected beam from the rear surface thereof. This exerts a large influence on a focus detection accuracy. Hence, the surface of the liquid crystal substrate can not be accurately detected in an in-focus state. This therefore leads to a first problem wherein a defective product can not be exactly distinguished.
Further, in the conventional focus detecting apparatus described above, if the detected surface has a substantially uniform distribution of reflectivity as seen on a specular surface, a position of the detected surface is detectable with a high accuracy. However, in a conventional focal position detecting apparatus, there is detected a position of the surface formed with, e.g., patterns having a distribution of reflectivity (surface of the liquid crystal substrate or the like, on which predetermined patterns are formed). In this case, there arises a second problem of causing a remarkable decline in terms of a focusing accuracy due to a shift of the center of optical gravity of the detection light because of a non-uniform distribution of reflectivity on the surface when a magnification of the objective lens is low.
Under such circumstances, it is a primary object of the present invention, which obviates at least one of the first and second problems, to provide a position detecting apparatus capable of executing a positional detection of the detected surface with a high accuracy.
Then, it is one secondary object of this invention, which obviates the first problem, to provide a position detecting apparatus capable of detecting a position of the substrate surface with a high accuracy even when the reflected light appears from a rear surface of the substrate including a liquid crystal substrate.
It is another secondary object of this invention, which obviates the second problem, to provide a position detecting apparatus capable of executing a positional detection of the detected surface having a non-uniform distribution of reflectivity with a high accuracy even when a magnification of the objective lens is low.
Additionally, it is still another secondary object of this invention, which simultaneously obviates the first and second problems, to provide a position detecting apparatus capable of detecting a position of the substrate surface (detected surface) with a high accuracy even when the unnecessary reflected light is present from the surface of the substrate as a detected object and when the surface of the substrate as the detected object has a non-uniform distribution of reflectivity.